Salinity is a significant agricultural problem, often associated with irrigation in arid or semi arid regions, particularly where there is inadequate subsurface drainage of excess rain and irrigation water. Irrigation water carries with it salts and where these are not drained, there remains a build up of salt after evaporation, or drainage of saline water into depressions.
Significant areas of the world are affected and in 1980 FAO/UNESCO estimated that there were about 3,230,000 km2 of saline land world wide, and for each continent this was estimated as follows (in106 ha); Australia, 84.7, Africa, 69.5, Latin America, 59.4, Near and Middle East, 53.1, Europe, 20.7, Asia and Far East, 19.5, Northern America, 16.0.
The United States Department of Agriculture estimates that, worldwide, 10 million hectares of arable land is lost to irrigation salinity every year. In Australia approximately 2.4 million hectares of land is affected by salinity and 5.7 million hectares of productive land is at risk. It has been estimated that the area of salt-affected land in Australia could increase six-fold in the next 30 to 50 years.
Engineering solutions such as reclamation, drainage and improved irrigation practices can reduce the severity and spread of salinization in many regions but costs of these practices are generally considered prohibitive.
An alternative approach for marginally affected land is to develop plants that are more salt tolerant and economically useful that can still be used in compromised soil. The benefit of this approach is that otherwise non-productive land can be used commercially. It also has the additional benefit of slowing movement of saline drainage water, increasing evapotranspiration and reducing and maintaining watertable levels below the soil surface, thereby slowing further deterioration.
Lucerne (Medicago spp.) is a perennial legume having a deep tap root, a number of stems extend upward from a woody crown to a height of up to about a metre, and bearing an abundance of leaves. It is widely used as livestock feed. New stems develop when older ones mature or have been removed by cutting or grazing. Reproduction is mainly via cross-fertilization by pollinators such as bees, but self-pollination may also occur.
Lucerne is mainly tetraploid, with 32 chromosomes, although some less vigorous diploid species are also known. Lucerne species comprise ecotypes, being population complexes adapted to particular environments in which the species is found with sufficient variability in the population to enable adaptation to change within a range of environmental parameters.
Apart from in the tropics, lucerne is widely adapted to temperature and soil conditions, however, all commercially available cultivars have poor salt tolerance. Being a widely used forage species, it is desirable to have a salt tolerant lucerne because there are large areas of potential forage pasture that could be developed but for the lack of a salt tolerant lucerne.
Whilst variations between ecotypes do exist, in general, forage yield of lucerne decreases 7.3% for each dSm−1 (˜11 mM NaCl) increase above a threshold of 2.0 dSm−1 (˜22 mM NaCl) (Johnson et al 1992 “Genetic and phenotypic relationships in response to NaCl at different development states of alfalfa” Thert Appl Genet Volume 83, Numbers 6-7, 833-838). Seedling lucerne yield is decreased by 50% at 8.9 dSm−1 (˜97 mM NaCl) (Mass & Hoffman, 1977—Crop salt tolerance current assessment J. Irrig Drainage Div. Am. Soc. Civil Eng. 103:115-134). In addition to variation between lucerne ecotypes, these values are dependent on soil conditions, so that on less well-drained soil the salt tolerance limit of lucerne plants may be lower.
Salinity research in lucerne has focussed in large part on seed germination and seedling establishment in the presence of NaCl. This research is however not representative of the natural habitat of most agricultural lucerne plants (Peel et al., (2004) Crop Sci 44:2049-2053). Less research has been conducted on persistence, yield, grazing tolerance, or other measures of agronomic suitability of mature lucerne plants in a saline environment and none that the inventor is aware of where the grazing tolerance has been assessed at high NaCl levels of 150 mM NaCl (about 1.37 dS/m), or greater.
There have been a number of attempts at developing salt tolerant lucerne plants. One variety resulting from such research is known as “Salado” (available from America's Alfalfa, Nampa, Id., USA) details of which are described in US 6005165.
The difficulty with this variety however is that whilst the seed is described as being capable of germinating at high salt levels, germination is only one aspect exhibited by a truly salt tolerant plant. Germination is very much more relevant for forage plants that are annual and self seeding, whereas for lucerne which is a perennial plant the self seeding is of less importance, and other characteristics such as its persistence and grazing resistance while growing in saline conditions are more critical. The “Salado” variety of lucerne was introduced on the Australian market in late November 1999 but has not had much commercial success because of reduced yields and lack of significantly higher salt tolerance when exposed to a rising salt level (approximately 2 dS/m per week) as established plants in comparison with other varieties not marketed as salt tolerant (see Peel et al., (2004)).
Another salt tolerant lucerne is described in U.S. Pat. No. 7,067,721 being a winter dormant variety, but no corresponding commercial plant has been marketed. The salinity tolerance of this lucerne population was tested at 115 mM NaCl.
Neither Peel et al., nor U.S. Pat. No. 7067721 describe the growth of lucerne in high saline environments for extended periods of time, nor do these documents show, under the same conditions, the recovery of lucerne by regrowth after cutting off mature stem growth. Also it is essential that a salt tolerant lucerne be palatable to livestock.
Salinity is a soil condition characterized by a high concentration of soluble salts. Soils are classified as saline when the ECe is 4 dS/m or more (131-USDA-ARS. 2008. Research Databases. Bibliography on Salt Tolerance. George E. Brown, Jr. Salinity Lab. US Dep. Agric., Agric. Res. Serv. Riverside, Calif.,) which is equivalent to approximately 40 mM NaCl or more and generates an osmotic pressure of approximately 0.2 MPa. This definition of salinity derives from the ECe that significantly reduces the yield of most crops.
The effect of salt on plants is multivarious and complex but the effects are usually assigned to two general categories. Firstly there is an osmotic effect, whereby plants are unable to take up sufficient water. Secondly there is a toxicity effect whereby the dominant ion, typically Na but also CI impact on particular metabolic and physiological processes.
Several cellular and plant mechanism are affected by salinity, these have been reviewed in Munns and Tester (2008 Annu Rev Plant Biol 59:651-681). The particular plant processes where salinity might impact or salinity tolerance arise might be summarised as follows 1) sensing and signalling in roots, 2) shoot growth, 3) photosynthesis, 4) accumulation of sodium in shoots, 5) accumulation of sodium in vacuoles and 6) accumulation of organic solutes.
It might reasonably be expected that altered genes or mutations in the complex genetics of any of these processes will impact on salt tolerance, and very likely that several of these together might provide cumulative effects and thus enhance tolerance. A simple genetic alteration would not be expected to result in enhanced tolerance.
Problems associated with salinity in agriculture have been experienced for millenia, and where cultivable land had become short suitable efforts to breed salt tolerant plants using conventional means have been attempted. There has been limited success with this and the levels of tolerance achieved are limited. This result might be expected because of the complexity particularly of the toxicity effects of the high levels of salts as referred to above, and the polyploid nature of many agricultural plants.
In more recent times attempts have been made using recombinant means particularly by altering the transport of salts, but also by altering the targets alleviating the effects and addressing quite a range of targets to do so. This recombinant approach is generally considered to hold out most promise.